This invention relates to a method for cooling melted thermoplastic resins, and more particularly to a method for direct cooling a melted thermoplastic resin being under a continuous extrusion operation in a continuous extruder.
Hitherto, as a cooling method for the melted thermoplastic resins being under a continuous extrusion operation, the indirect cooling through a cylinder and a screw has been used.
On this outside cooling method, cooling capacity is limited because of the small film coefficient of heat transfer at the side of the resins.
When the resin is mixed and kneaded to improve its quality or it is extruded at medium shearing speed of more than 50 sec.sup.-1, the internal heat due to its viscosity rises over the cooling capacity or becomes into an equibrium state, so that it is very difficult to cool the resin below the temperature of the resin flowing to the cooling region.
Therefore, the temperature of the resin flowing to the mix-kneading region of the continuous extruding machine rises too high and sufficient kneading cannot be obtained in the mix-kneading region, or else the temperature of the resin in the mix-kneading region or at the outlet of the discharge portion rises over the permitted temperature, resulting to inferior quality of the resin. These phenomena are serious defects of the method.